I solved this problem, but I just want to make a thread for anyone looking for a way to solve it.
I had to reinstall Ubuntu 11.04 Natty Narwhal today (long story) and after I installed the Ubuntu Restricted Extras from Ubuntu Software Manager, Flash still wasn't working. I use Google Chrome, which has its own Flash, so it was odd that Flash wasn't working (maybe it's different in Ubuntu; I don't know). I tried uninstalling and reinstalling in Synaptic, but that didn't work.
I also tried installing the ppa for the Flash 11 Beta plugin (http://www.ubuntugeek.com/how-to-ins...using-ppa.html), but this didn't work either.
Then I realized that the package in the Ubuntu repository was Flash 10, the old version; the current version is Flash 11. What I finally had to do was uninstall the bad Flash plugin through Ubuntu Software Center (or Synaptic), and then go here under FIREFOX (won't work in Google Chrome):
I downloaded the "APT for Ubuntu 10.04+" and chose to open this in Ubuntu Software Center. It said that this was a third party source with an unknown license, but this is probably just because Ubuntu hasn't upgraded from the old Flash 10 yet.
This whole issue was a bit ridiculous.